Since someone on here asked about the theme music that's played on startup, it's a preset and demo on my Korg Karma synthesizer keyboard called "Isle of Indigo". The synth is a specialized member of the Korg Triton series, where the name "Triton Center" comes from.
Here is an old demo I made of that preset on the website, it's from 2014:
https://www.skyscrapersim.net/downloads/other/isle_of_indigo.mp3In the source code repository, the file was added on June 12, 2006 as intro.mp3, later changed to intro.ogg on March 23, 2009 (that's when sound support was added to the C++ version).
But the original file, intro.wav, was created for the original development series before Skyscraper 1.0, and the timestamp is October 6, 2003 (Skyscraper 1.0 was from mid-2004, and includes it as intro.mp3).
I also have music I've made on that same synthesizer, if you were interested in hearing it, I might make a separate post about it.